Mobile phone cameras have improved a lot, and even my cheap Huawei can take useful pictures. They are limited by the lens and sensor size, and it is only recently that depth of field control has been introduced. A modern phone with 3 lens and advanced features is going to cost well into 4 figures, and that is above the budget for this project. Whilst some have received large payments for shots taken with a mobile, they tend to be opportunistic rather than planned stock photos. To build a portfolio of several thousand images, and to differentiate your photos from the millions of others that are available, I believe than one needs to have a fair degree of control over the image capturing process, and also it is useful to store them in RAW. This helps with the post processing.
Even thought many of the high end phones might cost in the 4 figures, lots of mobile service providers offer these phones on contract as a bundled item and you have the option to pay off that over 2 to 3 years. Why should people settle for inferior cameras, if you can get a excellent camera with a mobile phone?
The thing with companies like Shutterstock and Getty Images is that you are competing with professional photographers and they only pay for the ones that are exceptional.
I have a professional photographer in the family and loads of his photos gets no payments.
Do you have the secret sauce to share for the amateur photographers to make a little money?